A two-stroke internal combustion engine mounted on a handheld engine-driven working machine has a piston disposed in a cylinder and connected to a crankshaft, an ignition plug disposed in an upper portion of the cylinder, and an ignition control device activating the ignition plug. Effective activation of the ignition plug ignites fuel-air mixture in the cylinder to combust it, and inflation of the combusted air gives a force to the piston so as to move the piston from a top dead center position to a bottom dead center position. The ignition control device can set an ignition timing of the ignition plug with respect to the top dead center position of the piston.
In the two-stroke internal combustion engine, the ignition plug is normally activated for each rotation of the crankshaft. Further, the ignition timing of the ignition plug is advanced from the top dead center position of the piston. This is because there is a time lag from ignition of fuel-air mixture in the cylinder to reach of combustion of the mixture to the piston to give a force to the piston. Generally, the more the rotational speed increases, the larger the advanced angle is.
In a high rotational speed range, however, a state in which the ignition timing of the ignition plug is advanced from the top dead center position largely is maintained, the force given to the piston accelerates the internal combustion engine, so that the rotational speed of the internal combustion engine continues to increase. Thus, the ignition control device retards the ignition timing of the ignition plug in the rotational speed range, for example, to near the top dead center position. This decreases the force given to the piston, and prevents the rotational speed of the internal combustion engine from continuing to increase (for example, see Patent Publication 1).